This invent ion relates to a receiver, and more particularly to a radio receiver using a selective diversity receiving system.
Hitherto, as the receiving system using a plurality of antennas to provide an optimum receiving state, the diversity receiving system is well known. For the diversity receiving system, there are systems such as the selective diversity system, a switching diversity system, and the like. Explanation will now be given by taking an example of the selective diversity system.
In accordance with the selective diversity system, a plurality of receiving systems connected to respective antennas are sequentially selected by a selector to use, as a receiving signal, an output from a receiving system in the most satisfactory receiving state.
An example of the receiver of the conventional selective diversity system will now be described. The receiver roughly comprises two receiving systems, a selector for selecting any one of outputs from these two receiving systems, and a diversity controller for controlling the selector.
Each receiving system comprises a receiving circuit connected to an antenna to process an RF (Radio Frequency) signal to output a received signal, and a receiving level detection circuit for detecting a receiving level signal (e.g., an S-meter level signal) corresponding to a field strength to output a receiving level signal.
In each receiving system, the receiving circuit receives an RF signal from the antenna to output a received signal to the selector. The receiving level detection circuit detects a receiving level, e.g., from an output signal from circuits succeeding to a detector of the receiving circuit to output a detection signal to the diversity controller as a receiving level signal.
The diversity controller outputs, to the selector, a selector control signal for selecting either of receiving systems on the basis of respective receiving level signals. The selector selects either of received output signals on the basis of the selector control signal to output a selected one as a selected received output signal. Thus, the selector outputs an output signal corresponding to a receiving system in the most satisfactory receiving state.
In this way, the receiver selects a receiving system in a more satisfactory receiving state from these two receiving systems to maintain a satisfactory receiving state at all times.
However, the above-mentioned diversity receiver has a problem that selection of the receiving system is not necessarily precisely conducted.
Namely, the diversity controller generates a selector control signal either on the basis of a receiving level signal, or on the basis of a noise level signal, and both the received signal and the noise level are not considered at the same time. As a result, there might occur inconveniences such that if the receiving level is high, the received signal is selected even in the case where the level of the noise signal is high, so an optimum receiving system is not necessarily selected.
On the other hand, when attention is drawn to the internal problem of the receiving system, there are instances where there may occur small differences in the power gain of the front ends of the two respective receiving systems. To compensate such an unevenness in the operating characteristic (amplification degree), it is conceivable to use a gain controller. However, using the gain controller at high frequency circuits such as a front end, etc. is not preferable in view of generation of noise.